Aluminium electrowinning with metal-based anodes

ABSTRACT

A process for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte in a cell operating at reduced temperature, typically below 870° C., utilising nickel-alloy based anodes, in particular nickel-iron alloy anodes. The electrolyte contains AlF 3  in such a high concentration, usually above 20 weight %, in addition to cryolite, that fluorine-containing ions rather than oxygen ions are oxidised on the anodes. However, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the anodes. The anodes may be porous at the surface so as to provide a high active surface area for operation at low current density.

FIELD OF THE INVENTION

[0001] This invention relates to a process and cell for theelectrowinning of aluminium from alumina dissolved in afluoride-containing molten electrolyte using non-carbon, metal-basedanodes.

BACKGROUND ART

[0002] The production of aluminium since Hall and Heroult has beencarried out by dissolving the feed material consisting of pure aluminaobtained from bauxite in a cryolite-based electrolyte at about 950° C.This process has not evolved for more than one hundred years as manyother electrochemical processes.

[0003] Different types of carbon have been used as anode, cathode andsidewall material. All attempts to utilise other materials have failedwith the exception of silicon carbide for sidewalls and more recentlyTiB₂ protective coatings on carbon cathodes instead of or in addition toa thick pool of aluminium protecting the cathodes against cryoliteattack.

[0004] The carbonaceous anodes must be replaced every few weeks. Duringelectrolysis the oxygen which should evolve on the anode surfacecombines with the carbon to form polluting CO₂ and small amounts of COand fluorine-containing dangerous gases. The actual consumption of theanode is as much as 450 Kg/Ton of aluminium produced which is more than⅓ higher than the theoretical amount of 333 Kg/Ton.

[0005] Using metal anodes in aluminium electrowinning cells woulddrastically improve the aluminium process by reducing pollution and thecost of aluminium production.

[0006] U.S. Pat. No. 4,374,050 (Ray) discloses inert anodes made ofspecific multiple metal compounds which are produced by mixing powdersof the metals or their compounds in given ratios followed by pressingand sintering, or alternatively by plasma spraying the powders onto ananode substrate. The possibility of obtaining the specific metalcompounds from an alloy containing the metals is mentioned.

[0007] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describesnon-carbon anodes for aluminium electrowinning coated with a protectivecoating of cerium oxyfluoride, formed in-situ in the cell orpre-applied, this coating being maintained by the addition of a ceriumcompound to the molten cryolite electrolyte. This made it possible tohave a protection of the surface from the electrolyte attack and to acertain extent from the gaseous oxygen but not from the nascentmonoatomic oxygen.

[0008] EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describesanodes composed of a chromium, nickel, cobalt and/or iron basedsubstrate covered with an oxygen barrier layer and a ceramic coating ofnickel, copper and/or manganese oxide which may be further covered withan in-situ formed protective cerium oxyfluoride layer. Likewise, U.S.Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan)disclose aluminium production anodes with an oxidised copper-nickelsurface on an alloy substrate with a protective oxygen barrier layer.However, full protection of the alloy substrate was difficult toachieve.

[0009] U.S. Pat. No. 4,681,671 (Duruz) discloses aluminium productionfrom alumina dissolved in an electrolyte between 680° and 690° C. in acell utilising metal anodes that have an electrochemically activesurface whose area is increased at least 5 times compared toconventional anodes. The anodes are arranged for the discharge of oxideions preferentially to fluorine ions using a low current density at theanode. Use of such a process with a multimonopolar arrangement ofnon-consumable electrodes that are vertical or at a slope, is describedin U.S. Pat. No. 5,725,744 (Duruz/de Nora).

[0010] In Belyaev & Studentsov: Electrolysis of Alumina in FusedCryolite with Oxide Anodes, Legkie Metali 6 No. 3, 1937, pp. 17-24 andBelyaev: Electrolysis of Alumina with Ferrite Anodes, Legkie Metali 7No. 1, 1938, pp. 7-20, it has been established in tests using anodesmade of precious metals such as platinum, and bulk ceramic oxides suchas ferrites that the primary anodic product resulting from theelectrolysis of cryolite-alumina melts is oxygen.

[0011] Metal or metal-based anodes are highly desirable in aluminiumelectrowinning cells instead of carbon-based anodes. Many attempts weremade to use metallic anodes for aluminium production, however they werenever adopted by the aluminium industry because they had a short lifeand contaminated the aluminium produced.

[0012] All efforts made to utilise non-carbon anodes and avoid pollutionby CO₂ and organic fluorides have not succeeded because all non-noblemetal oxides, which are the only materials commercially acceptable andresistant to oxygen, are more or less soluble in cryolite which waschosen and is still used as the electrolyte because it is a good solventof oxides such as alumina.

OBJECTS OF THE INVENTION

[0013] An object of the invention is to provide a process and cell foraluminium electrowinning using long-lasting non-carbon anodes so as toeliminate carbon-generated pollution.

[0014] Another object of the invention is to provide a process and cellfor aluminium electrowinning using metal-based anodes, in which theconditions are such as to inhibit corrosion or oxidation of the anodes.

[0015] A further object of the invention is to provide an aluminiumelectrowinning process and cell with anodes having a highelectrochemical activity and a low or no solubility in the electrolyte.

[0016] Another object of the invention is to provide an aluminiumelectrowinning process and cell utilising improved metal-based anodesmade of readily available material(s).

[0017] A major object of the invention is to provide an aluminiumelectrowinning process and cell using metal anodes and operating undersuch conditions that the contamination of the product aluminium islimited.

SUMMARY OF THE INVENTION

[0018] The present invention concerns an aluminium electrowinningprocess in a cell containing alumina dissolved in a fluoride-basedmolten electrolyte and utilising specific metal alloy-based anodes whichdo not require to be made of oxides in order to be electrochemicallyactive and resistant to the attack of the molten electrolyte and ofoxygen gas.

[0019] Several models of anodic reactions can be considered to explainthe production of oxygen gas during the electrowinning process of theinvention, namely:

2O²⁻−4e=C   [1]

2AlO₃ ³⁻−6e=Al₂O₃+3/2O₂   [2]

2AlO₂ ⁻−2e=Al₂O₃+1/2O₂   [3]

2F⁻−2e=F₂;

and

2Al₂O₃+6F₂=4AlF₃+O₂  [4]

F⁻ −e=F;

and

Al₂O₃+6F=2AlF₃+O;

and

O+O=O₂  [5]

2AlF₆ ³⁻+Al₂O₃−6e=2Al₂F₆+3/2O₂   [6]

2AlF₃+Al₂O₃=Al₂F₆O₂ ⁻+Al³⁺

or

2AlF₃+AlO₂ ⁻=Al₂F₆O²⁻+Al³⁺

or

2AlF₃+O²⁻=Al₂F₆O²⁻;

and

Al₂F₆O²⁻−2e=Al₂F₆O;

and

Al₂F₆O=Al₂F₆+½O₂  [7]

[0020] Whereas mechanisms [1] to [7] have been defined in terms ofstoichiometric compounds, it is possible that corresponding mechanismsinvolving non-stoichiometric compounds may occur during electrolysis.

[0021] The present invention is based on the observation that underspecific cell operating conditions, i.e. reduced electrolysistemperature and high fluoride content in the electrolyte, theelectrochemical oxidation reaction of oxygen ions or fluorine-free ionicoxides to form oxygen gas, i.e. reactions [1] to [3], can be minimisedor even suppressed. Hence, the oxidation of fluorine ions or ionicfluorine-containing compounds, i.e. reactions [4], [5], [6] and [7], inparticular the reaction involving the oxidation of F⁻ to nascentfluorine F and/or of aluminium oxyfluoride ions [7], become the main oronly electrochemical reactions occurring on the electrochemically activeanode surface. This inhibits direct contact of reactive oxygen species,in particular nascent monoatomic oxygen, with the electrochemicallyactive surface, which greatly reduces the risk of oxidation andcorrosion of the anode by these oxygen species.

[0022] Furthermore, it has been observed that nickel alloys, inparticular nickel-iron metal alloys, are electrochemically active with alow overvoltage for the oxidation of fluorine ions or ionicfluorine-containing compounds such as aluminium oxyfluoride ions and,surprisingly, are stable and substantially do not react with the productof the anodic electrolysis even after several hundred hours ofelectrolysis under specific cell operating conditions.

[0023] The anodes used in this invention consist essentially of a nickelalloy, in particular of a nickel-iron based alloy, and can be used assuch for efficient and successful operation in a melt having a highconcentration of aluminium fluoride and operated at reduced temperature.

[0024] Cermet anodes which have been described in the past in relationto aluminium production have an oxide content which forms the majorphase of the anode. Conversely, the anode according to the invention ismade predominantly of metal, possibly covered with a thin oxide layer.For the first time, this invention permits utilisation of a non-noblemetal anode which is resistant to a fluoride-based molten electrolyte,electrochemically active and has a very long life the limit of which hasnot been determined yet.

[0025] The invention relates to a process for the electrowinning ofaluminium from alumina dissolved in a fluoride-based molten electrolytein a cell operating at reduced temperature and utilising metal-basedanodes. The anodes comprise an alloy of nickel and an alloying metal, inparticular iron, having an outer part consisting predominantly of nickelwhich forms an electrochemically active surface for the oxidation ofions. In this process the electrolyte contains AlF₃ in such a highconcentration that fluorine-containing ions, such as aluminiumoxyfluoride ions, predominantly rather than oxygen ions are oxidised onthe electrochemically active surfaces. However, only oxygen is evolved,the evolved oxygen being derived from the dissolved alumina present nearthe electrochemically active anode surfaces.

[0026] As in the fluorine oxidation reactions [4], [5], [6] and [7]listed above, the oxidation of fluorine-containing ions covers oxidationof ions of fluorine as such as well as ions contained in a fluorinecompound such as AlF₆ ³⁻ or Al₂F₆O²⁻.

[0027] As explained below, the outer part of the nickel alloyadvantageously has an open porosity defining a high surface areaelectrochemically active surface. The total amount of electrolysiscurrent passed between the anode and facing cathode which corresponds toabout to 0.5 to 1.5 A/cm² at the cathode surface of an industrial cellcorresponds to a lower current density on the high surface areaelectrochemically active surface. The actual current density on thesurface of the pores of the anode is typically 5 to 50 times smallerthan the corresponding density on the cathode.

[0028] To prevent anode effects and corrosion of the anode byfluorine-containing ions oxidised on the electrochemically active anodesurface, a sufficient concentration of dissolved alumina is permanentlypresent in the molten electrolyte near the electrochemically activeanode surfaces so that fluorine-containing ions react before or aftertheir oxidation with oxygen ions from the dissolved alumina to evolveoxygen gas instead of fluorine.

[0029] The cell is preferably operated with a crustless and ledgelesselectrolyte, as described in co-pending application PCT/IB99/01739 (deNora/Duruz). To ensure sufficient dissolution of alumina in theelectrolyte at reduced temperature, the cell is preferably fitted withan alumina spraying device to spray and distribute alumina oversubstantially the entire surface of the molten electrolyte, as disclosedin PCT/IB99/00697 (de Nora/Berclaz). To promote circulation of moltenelectrolyte rich in dissolved alumina to the electrochemically activeanode surface, the electrodes may be designed as disclosed in WO99/41429(de Nora/Duruz) and in PCT/IB99/01740 (de Nora). Preferably, the anodeshave a foraminate electrochemically active structure to permitcirculation of the molten electrolyte therethrough, as disclosed inPCT/IB99/00018 (de Nora), which is advantageously fitted with afunnel-like arrangement to guide the molten electrolyte from and to theelectrochemically active anode surfaces as disclosed in PCT/IB99/00017(de Nora).

[0030] Normally, the molten electrolyte contains cryolite and, inaddition to cryolite, an excess of AlF₃ in an amount of at least 20weight % of the electrolyte typically 23 weight % or more, preferablybetween 25 and 35 weight %, in particular between 27 to 30 weight %, forexample about 28 weight % of the electrolyte. The electrolyte mayfurther contain CaF₂ and/or MgF₂.

[0031] The reduced temperature of the molten electrolyte should be at900° C. or 910° C. at the most, typically below 880° C. and preferablybelow 870° C., and above the melting point of aluminium, but usuallyabove 730° C.

[0032] As stated above, the cell may advantageously be fitted with meansto circulate electrolyte containing dissolved alumina to constantlymaintain a sufficient concentration of dissolved alumina near theelectrochemically active anode surfaces.

[0033] The invention also relates to a cell for the electrowinningaluminium from alumina dissolved in a fluoride-based molten electrolyteoperating at reduced temperature and utilising metal-based anodes. Theanodes comprise an alloy of nickel and an alloying metal, in particulariron, having an outer part consisting predominantly of nickel whichforms an electrochemically active surface for the oxidation of ions. Theelectrolyte contains AlF₃ in such a high concentration thatfluorine-containing ions, such as aluminium oxyfluoride ions,predominantly rather than oxygen ions are oxidised on theelectrochemically active surfaces, but only oxygen is evolved, theevolved oxygen being derived from the dissolved alumina present near theelectrochemically active anode surfaces.

[0034] Preferably, aluminium is produced on an aluminium-wettablecathode, in particular on a drained cathode, for instance as disclosedin U.S. Pat. No. 5,683,559 (de Nora) or in PCT application WO99/02764(de Nora/Duruz).

[0035] In one embodiment of the cell, each anode is a nickel-iron alloybased anode. The anode before use has an electrochemically activesurface with an oxide film. When it is polarised in a molten electrolyteof a cell, it becomes electrochemically active for the oxidation offluorine ions rather than oxygen ions. However, only oxygen is evolvedwhich is derived from the dissolved alumina present near theelectrochemically active anode surfaces.

[0036] Before use, the alloy of which the anode is made may have aNi/Fe, or more generally nickel/alloying metal, atomic ratio below 1.Alternatively, the Ni/Fe atomic ratio may be at least 1, in particularfrom 1 to 4. As described below, when the outer part of the anode ismade porous by oxidation and removal of the alloying metal, a highercontent of alloying metal leads to a greater porosity whereas a lowercontent of alloying metal leads to a smaller removal and formation of areduced porosity.

[0037] The alloy can further contain one or more additives. Before use,the alloy may contain nickel and the alloying metal, in particular iron,in a total amount of at least 85 weight %, in particular at least 95weight %, and the balance additive(s). For example, one or moreadditives can be selected from chromium, copper, cobalt, silicon,titanium, tantalum, tungsten, vanadium, yttrium, of at least 85 weight%, in particular at least 95 weight %, and the balance additive(s). Forexample, one or more additives can be selected from chromium, copper,cobalt, silicon, titanium, tantalum, tungsten, vanadium, yttrium,molybdenum, manganese, aluminium and niobium in a total amount of up to5 or even 10 weight % of the alloy before use. One or more additives maybe catalytically active for the desired reaction(s) and selected fromiridium, palladium, platinum, rhenium, rhodium, ruthenium, tin or zincmetals, Mischmetals and their oxides and metals of the Lanthanide seriesand their oxides as well as mixtures and compounds thereof in a totalamount of up to 5 weight % of the alloy before use.

[0038] The outer part of the anodes may comprise more than 75 weight %nickel, preferably between 85 and 95 weight % nickel.

[0039] The nickel metal rich outer part typically has a porositydefining a high surface area electrochemically active surface and whichcan be obtained by oxidation in an oxidising atmosphere before use.Usually, the porosity contains cavities which are partly or completelyfilled before use with nickel and/or iron oxides or more generallyoxides of nickel and/or the alloying metal and during use with one ormore fluorine-containing compounds of at least one metal selected fromnickel, iron or other alloying metal, and aluminium.

[0040] The porosity defining a high surface area electrochemicallyactive surface can alternatively be obtained or can be completed bydissolving part of the iron or other alloying metal into the electrolyteof the aluminium electrowinning cell, or of another electrolytic celland then transferred into the aluminium electrowinning cell, thisdissolution taking place usually soon after electrolysis start-up.During use, the porosity usually contains cavities which are partly orcompletely filled with fluorides of at least one metal selected fromnickel, iron or other alloying metal and aluminium.

[0041] In one embodiment the nickel alloy underlying theelectrochemically active surface has a decreasing concentration of ironor other alloying metal(s) towards the electrochemically active surfacelayer.

[0042] The nickel metal rich outer part can comprise nickel metal andiron or other alloying metal in a Ni/Fe or more generallynickel/alloying metal atomic ratio of more than 3 where it reaches theelectrochemically active surface.

[0043] A suitable nickel-iron alloy based anode for such a cell can beproduced as follows. A nickel alloy substrate, in particular anickel-iron alloy substrate, is heat treated in an oxidising atmosphereto form a nickel alloy based anode having an integral thin oxide filmand anodically polarised in a molten electrolyte contained in a cell asdescribed above, whereby fluorine-containing ions predominantly ratherthan oxygen ions are oxidised on the electrochemically active surface ofthe nickel-iron anode.

[0044] When the alloy is covered with a thin oxide film obtainable byoxidation before use, during use the oxides of nickel and iron or otheralloying metal present on and possibly in the alloy substrateoriginating from the oxidation treatment in the oxidising atmosphere maybe dissolved in the molten electrolyte without being replaced, or may besubstituted with one or more fluorine-containing compounds of aluminiumfrom the electrolyte and of iron and nickel from the anode.

[0045] The nickel-iron or other nickel alloy substrate can be heattreated in an oxidising atmosphere for 20 minutes to 5 hours or even 6hours, preferably 30 to 240 minutes, for example about 120 minutesduring use, at a temperature of 900 to 1200° C. It can be heat treatedin an oxidising atmosphere containing 10 to 100 molar % O₂ and thebalance one or more inert gases. The nickel-iron or other nickel alloysubstrate can also be heat treated in air.

[0046] After formation of the integral oxide film, the nickel-iron orother nickel alloy substrate may further be heat treated in an inertatmosphere.

[0047] As nickel and cobalt behave very similarly under the abovedescribed cell conditions, in a modification of the above aspects of theinvention, the nickel of the metal-based anodes, in particular of theirouter part, is wholly or predominantly substituted by cobalt. Forexample, the anode is made from a nickel-cobalt-iron alloy or acobalt-iron alloy, in which case its outer part is rich in nickel andcobalt metal, or rich in cobalt metal only, respectively.

[0048] The invention also relates to the use of a nickel alloy, inparticular a nickel-iron alloy, which comprises a surfaceelectrochemically active for the oxidation of fluorine ions as an anodeof a cell for the electrowinning of aluminium from alumina dissolved ina fluoride-based molten electrolyte. The electrochemically activesurface of the anode is a surface of the nickel alloy as such oroxidised before or during electrolysis.

DETAILED DESCRIPTION

[0049] The invention will be further described in the followingExamples:

EXAMPLE 1

[0050] Anode Preparation:

[0051] An anode suitable for producing aluminium according to theinvention was made by pre-oxidising in air at 1100° C. for 30 minutes asubstrate of a nickel-iron alloy consisting of 50 weight % nickel and 50weight % iron, to form a very thin oxide surface film on the alloy.

[0052] The surface oxidised anode was cut perpendicularly to the anodeoperative surface and the resulting section of the anode was subjectedto microscopic examination.

[0053] Before use, the anode had an external oxide surface layer havinga thickness of up to 20-25 micron. This layer in the given example of anickel-iron alloy consisted of an iron-rich nickel-iron oxide and,underneath, an iron-depleted nickel-iron alloy outer part containinggenerally round columnar pores filled with iron-rich nickel-iron oxide.The pores had a diameter of about 2 to 5 micron. The nickel-iron alloyof the outer part contained about 80-85 weight % nickel.

[0054] Underneath this outer part, the nickel-iron alloy had remainedsubstantially unchanged.

EXAMPLE 2

[0055] Electrolysis Testing:

[0056] An anode prepared as in Example 1 was tested in an aluminiumelectrowinning cell containing a molten electrolyte at 850° C.consisting essentially of NaF and AlF₃ in a weight ratio NaF/AlF₃ ofabout 0.7 to 0.8, i.e. an excess of AlF₃ in addition to cryolite ofabout 26 to 30 weight % of the electrolyte, and approximately 3 weight %alumina. The alumina concentration was maintained at a substantiallyconstant level throughout the test by adding alumina at a rate adjustedto compensate the cathodic aluminium reduction. The test was carried outat an apparent current density of about 0.6 A/cm² which generallycorresponds to a current density of less than about 0.06 A/cm² on thesurface of the pores. The electrical potential of the anode remainedsubstantially constant at 4.2 volts throughout the test.

[0057] During electrolysis aluminium was cathodically produced whilefluorine and/or fluorine-containing ions, such as aluminium oxyfluorideions, rather than oxygen ions were oxidised on the nickel-iron anodes.However, only oxygen was evolved which was derived from the dissolvedalumina present near the anodes.

[0058] After 72 hours, electrolysis was interrupted and the anode wasextracted from the cell. The external dimensions of the anode hadremained unchanged during the test and the anode showed no signs ofdamage.

[0059] The anode was cut perpendicularly to the anode operative surfaceand the resulting section of the anode was subjected to microscopicexamination, as in Example 1.

[0060] It was observed that the anode had an electrochemically activesurface covered with a discontinuous, macroporous, non adherent ironoxide layer of the order of between 500 to 1000 micron thick,hereinafter called the “excess iron oxide layer”. The excess iron oxidelayer was pervious to and contained molten electrolyte, indicating thatit had been formed during electrolysis.

[0061] The excess iron oxide layer resulted from the excess of ironcontained in the part of the nickel-iron alloy underlying theelectrochemically active surface and which diffuses therethrough. Inother words, the excess oxide layer resulted from an iron migration frominside to outside the anode during the electrolysis.

[0062] Such an iron oxide layer has no or little electrochemicalactivity. It slowly diffuses and dissolves into the electrolyte untilthe part of the anode underlying the electrochemically active surfacereaches an iron content of about 15-20 weight % corresponding to anequilibrium under the operating conditions at which iron ceases todiffuse, and thereafter the layer continues to dissolve into theelectrolyte.

[0063] The anode's aforesaid outer part had been transformed duringelectrolysis. Its thickness had grown from 20-25 micron to about 500 to1000 micron and the cavities had also grown in size to vermicular formbut were only partly filled with nickel and iron compounds. The cavitieshad a length of about 10 to 20 micron and a diameter of about 2 to 5micron. The nickel and iron oxides filling the cavities had beenfluorised to form fluoride-containing nickel and iron ceramic compounds.

[0064] The presence of the fluoride-containing nickel and iron ceramiccompounds attests the anodic fluorine reaction, i.e. mechanisms [4],[5], [6] and/or [7].

[0065] The cavities also contained aluminium fluoride but no electrolytewas detected and no sign of corrosive damage appeared throughout theanode.

[0066] Underneath the outer part, the nickel-iron alloy had remainedunchanged.

[0067] The shape and external dimensions of the anode remained unchangedafter electrolysis which demonstrated stability of this anode structureunder the operating conditions in the molten electrolyte.

[0068] In another test a similar anode was operated under the sameconditions for several hundred hours at a substantially constant currentand cell voltage which demonstrated the long anode life compared toknown non-carbon anodes.

EXAMPLE 3

[0069] Anode Preparation:

[0070] Another anode suitable for producing aluminium according to theinvention was prepared by coating a nickel-rich nickel-iron alloysubstrate with a layer of nickel-iron alloy richer in iron, and heattreating this coated substrate. The alloy substrate consisted of 80weight % nickel and 20 weight % iron. The alloy layer consisted of about50 weight % nickel and 50 weight % iron.

[0071] The alloy layer was electrodeposited onto the alloy substrateusing an appropriate electroplating bath prepared by dissolving thefollowing constituents in deionised water at a temperature of about 50°C.: a. Nickel sulfate hydrate (NiSO₄.7H₂O): 130 g/l b. Nickel chloridehydrate (NiCl₂.6H₂O): 90 g/l c. Ferrous sulfate hydrate (FeSO₄.78H₂O):52 g/l d. Boric acid H₃BO₃: 49 g/l e. 5-Sulfo-salicylic acid hydrate(C₇H₆O₆S.2H₂O): 5 g/l f. o-Benzoic acid sulfimide Sodium salt hydrate3.5 g/l (C₇H₄NaO₃S.aq): g. 1-Undecanesulfonic acid Sodium salt(C₁₁H₂₃NaO₃S): 3.5 g/l

[0072] To assist dissolution, the constituents were stirred in thedeionised water.

[0073] The alloy layer was electrodeposited onto the cathodicallypolarised alloy substrate from a nickel-iron alloy anode consisting of50 weight % nickel and 50 weight % iron, immersed in the electroplatingbath at a temperature of 50 to 55° C. After 4 hours electrodeposition ata cathodic current density of 0.060 A/cm², the deposited layer had anaverage thickness of about 250 to 280 micron with an average compositionof 47.5 weight % nickel and 52.5 weight % iron.

[0074] After deposition, the coated alloy substrate was surface oxidisedat 1100° C. in air for 1 hour and cooled to room temperature. Thesurface-oxidised anode was then cut perpendicularly to the anodeoperative surface and the resulting section of the anode was subjectedto microscopic examination as in Example 1.

[0075] It was observed that the external anode surface was covered withiron-rich nickel-iron oxides over a thickness of about 20 to 25 micron.

[0076] The alloy layer had an iron-depleted nickel-iron alloy outer partwith a thickness of about 50 micron, this outer part containingvermicular iron-rich nickel-iron oxide inclusions in a nickel-iron alloycontaining about 70 to 75 weight % nickel metal. Underneath this outerpart, the composition of the alloy layer had remained substantiallyunchanged.

[0077] Some minor interdiffusion of iron was also observed at theinterface between the alloy layer and the alloy substrate enhancing theadherence of the layer on the substrate.

EXAMPLE 4

[0078] Electrolysis Testing:

[0079] An anode prepared as in Example 3 was tested in an aluminiumelectrowinning cell as in Example 2 except that the electrolytecontained approximately 4 weight % alumina and that the anode was testedduring 75 hours.

[0080] During electrolysis aluminium was produced and oxygen evolved.The anode when inspected showed no signs of having been subjected to theusual type of oxidation/passivation mechanisms observed with prior artprocess. This lead to the conclusion that predominantly fluorine and/orfluorine-containing ions, such as aluminium oxyfluoride ions, ratherthan oxygen ions were oxidised on the nickel-iron anodes. However, onlyoxygen was evolved which was derived from the dissolved alumina presentnear the anodes.

[0081] After electrolysis the anode was extracted from the cell andexamined.

[0082] The external surfaces of the anode were crust free and itsexternal dimensions were practically unchanged. No sign of damage wasvisible.

[0083] The anode was cut perpendicularly to the operative surface andthe resulting section of the anode was subjected to the microscopicexamination as in Example 1.

[0084] It was observed that the anode surface was covered with an ironrich oxide over a thickness of less than 25 to 50 micron. The thinnessof this oxide layer attested the fact that the anode had not, or onlymarginally, been exposed to nascent monoatomic oxygen, hence that theoxidation process of fluorine-containing ions was predominant over theprocess of oxygen ions.

[0085] The anode's outer part (depleted in iron metal) had grown from 50to about 250 micron containing mainly empty pores. The pores werevermicular with a length limited to the thickness of the overall alloylayer and a diameter of about 10 micron. The outer part was furtherdepleted in iron metal and had a composition of about 75 weight % nickeland 25 weight % iron.

[0086] The structure and composition of the alloy substrate had remainedsubstantially unchanged, with the exception of empty pores of randomshape having a size of about 5 to 10 micron that were located at thesubstrate/layer interface and up to a depth of 100 to 150 micron. Theempty pores resulted from the internal oxidation and diffusion towardsthe anode's surface of iron during electrolysis.

EXAMPLE 5

[0087] Anode Preparation:

[0088] A metallic anode consisting of an alloy of 70 weight % nickel and30 weight % iron was conditioned to be suitable for electrolysisaccording to the invention by anodic polarisation in an electrolyticcell. The electrolytic cell contained a molten electrolyte at 850° C.consisting essentially of NaF and AlF₃ in a weight ratio NaF/AlF₃ ofabout 0.7 to 0.8, i.e. an excess of AlF₃ in addition to cryolite ofabout 26 to 30 weight % of the electrolyte. The electrolyte contained noalumina other than that included in impurities of the added AlF₃ makingabout 2 weight % of the electrolyte.

[0089] Before immersion into the electrolyte, the anode was pre-heatedfor 0.5 hour over the cell to a temperature of about 750° C.

[0090] After immersion into the conditioning electrolyte, the anode waspolarised at an initial current density of about 0.06-0.1 A/cm² whichdecreased over time to less than about 0.01 A/cm². The cell voltage wasabout 2.2 volt and the anode potential was below 2 volt Thus,substantially no oxygen could be evolved during polarisation. Thecurrent passed during polarisation was essentially due to selectiveanodic dissolution of iron present at and close to the surface of theanode.

[0091] After 24 hours, polarisation was interrupted and the anode wasextracted from the cell. The external dimensions of the anode hadremained unchanged and was covered with black oxide.

[0092] This conditioned anode was ready to be used for the production ofaluminium according to the invention. The anode's composition wasascertained by cutting it perpendicular to the operative surface and theresulting section of the anode was subjected to the microscopicexamination, as in Example 1.

[0093] It was observed that the anode surface was covered with a verythin film of iron-rich oxide having a thickness of less than 1 micron.Underneath, the anode had an outer iron-depleted nickel-iron alloy partwhich had an average thickness of 100 to 150 micron. This outer alloypart had vermicular pores with a diameter of 10 to 30 micron that wereempty except for small oxide inclusions.

[0094] The average metal composition of the outer alloy part was about80 weight % nickel and 20 weight % iron. Below the outer alloy part, theinitial nickel-iron alloy composition had remained substantiallyunchanged.

[0095] In a variation of this Example, the composition of the anode canbe changed. For instance, the starting alloy contains 30 weight % nickeland 70 weight % iron or 80 weight % nickel and 20 weight % iron.

[0096] A coated substrate as described in Example 3 can also beconditioned to form an anode suitable for the production of aluminiumaccording to the invention by dissolving part of the iron of the anodeas described in Example 5.

[0097] All or part of the nickel content of the anodes of Examples 1, 3and 5 can be replaced by cobalt.

EXAMPLE 6

[0098] Electrolysis Testing:

[0099] An anode as prepared in Example 5 was used in an aluminiumelectrowinning cell containing a molten electrolyte as described inExample 4.

[0100] As in Example 4, during electrolysis aluminium was produced andoxygen evolved. The anode inspection also led to the conclusion thatfluorine-containing ions predominantly rather than oxygen ions wereoxidised on the anode surface.

[0101] After 75 hours, electrolysis was interrupted and the anode wasextracted from the cell. The external surfaces of the anode were crustfree and its external dimensions were practically unchanged. No sign ofdamage was visible.

[0102] The anode was cut perpendicularly to the operative surface andthe resulting section of the anode was subjected to the microscopicexamination as in Example 1.

[0103] It was observed that the anode surface was covered with a ironrich oxide over a thickness of less than 25 to 50 micron. The anodesurface was covered by a very thin film of iron-rich oxide having athickness of less than 100 micron, which indicated that the irondepletion during electrolysis was less than for a pre-oxidised anode asin Example 2.

[0104] The anode outer part had grown from 150 micron to about 500 to750 micron and contained pores that were substantially empty in theirmajority. Below this outer part, the alloy composition had remainedunchanged.

EXAMPLE 7

[0105] Anode Construction and Electrolysis Testing:

[0106] An anode having an active structure of 210 mm diameter was madeof three concentric rings spaced from one another by gaps of 6 mm. Therings had a generally triangular cross-section with a base of about 19mm and were connected to one another and to a central vertical currentsupply rod by six members extending radially from the vertical rod andequally spaced apart from one another around the vertical rod. The gapswere covered with chimneys for guiding the escape of anodically evolvedgas to promote the circulation of electrolyte and enhance thedissolution of alumina in the electrolyte as disclosed in PCTpublication WO00/40781 (de Nora).

[0107] The anode and the chimneys were made from cast nickel-iron alloycontaining 50 weight % nickel and 50 weight % iron that was heat treatedas in Example 1. The anode was then tested in a laboratory scale cellcontaining an electrolyte as described in Example 2 except that itcontained approximately 4 weight % alumina.

[0108] During the test, a current of approximately 280 A was passedthrough the anode at an apparent current density of about 0.8 A/cm² onthe apparent surface of the anode which generally corresponds to acurrent density of less than about 0.08 A/cm² on the surface of thecolumnar pores of the anode. The electrical potential of the anoderemained substantially constant at approximately 4.2 volts throughoutthe test.

[0109] The electrolyte was periodically replenished with alumina tomaintain the alumina content in the electrolyte close to saturation.Every 100 seconds an amount of about 5 g of fine alumina powder was fedto the electrolyte. The alumina feed was periodically adjusted to thealumina consumption based on the cathode efficiency, which was about67%.

[0110] As in Examples 4 and 6, during electrolysis aluminium wasproduced and oxygen evolved. The anode inspection also led to theconclusion that fluorine-containing ions predominantly rather thanoxygen ions were oxidised on the anode surface.

[0111] After more than 1000 hours, i.e. 42 days, electrolysis wasinterrupted and the anode was extracted from the cell and allowed tocool. The external dimensions of the anode had not been substantiallymodified during the test but the anode was covered with iron-rich oxideand bath. The anode showed no sign of damage.

[0112] The anode was cut perpendicularly to the anode operative surfaceand the resulting section of a ring of the active structure wassubjected to microscopic examination, as in Example 1.

[0113] It was observed that the porous outer alloy part had grown insidethe anode ring to a depth of about 7 mm leaving only an inner part ofabout 5 mm diameter unchanged, i.e. consisting of a non-porous alloy of50 weight % nickel and 50 weight % iron. The outer porous alloy part ofthe anode had a concentration of nickel varying from 85 to 90 weight %at the anode surface to 70 to 75 weight % nickel close to the non-porousinner part, the balance being iron. The iron depletion in the porousalloy outer part corresponded about to the accumulation of iron presentas oxide on the surface of the anode, which indicated that the ironoxide had not substantially dissolved into the electrolyte during thetest.

SUMMARY OF EXAMPLES

[0114] In summary, the analysis of the anodes tested in all the aboveExamples showed that, at equal anode current, the oxidation rate ofnickel-alloy anodes was between about 20 and 100 times smaller than theoxidation rate under conventional conditions in which the oxidation ofoxygen ions is the sole or the predominant mechanism occurring at thesurface of the anode, so in the above described Examples thenickel-alloy anodes should last several thousand hours, whereas in anormal cryolite electrolyte the anodes last less than 50 hours.

[0115] It is believed that the greatly reduced oxidation of iron at theanode surface under the present electrolysis conditions can have twocauses. The first possible cause of oxidation is exposure to nascentoxygen produced by the oxidation of oxygen ions at the anode surfacewhich may marginally occur in parallel to the oxidation offluorine-containing ions and which might represent less than 1% of theoverall oxidation mechanism at the anode surface. The second cause ofoxidation is exposure to dissolved molecular oxygen which is marginallypresent in the electrolyte at a theoretical pressure of about 10⁻¹⁰ atmunder the test conditions.

[0116] If the surface of nickel-iron alloy anodes described above wereexposed to significant oxygen concentration in the electrolyte, thenickel of the anode would be rapidly oxidised into NiO which wouldpassivate the anode and prevent electrolysis. The absence of suchoxidation/passivation confirms that no or substantially no oxygen ionsare oxidised at the surface of the nickel-alloy anodes.

[0117] In addition, the presence of sodium-free fluorides, such asnickel, iron and aluminium fluorides and oxyfluorides, was observed inthe pores of the tested anodes. This indicates that not electrolyte butfluorine or fluorides from the active anode surface penetrated intothese pores, and confirms that the mechanism of oxidation offluorine-containing ions took place at the surface of the anodes.

1. A process for the electrowinning of aluminium from alumina dissolvedin a fluoride-based molten electrolyte in a cell operating at reducedtemperature and utilising metal-based anodes comprising an alloy ofnickel and an alloying metal having an outer part consistingpredominantly of nickel which forms an electrochemically active surfacefor the oxidation of ions, in which the electrolyte contains AlF₃ insuch a high concentration that fluorine-containing ions predominantlyrather than oxygen ions are oxidised on the electrochemically activesurfaces, however, only oxygen is evolved, the evolved oxygen beingderived from the dissolved alumina present near the electrochemicallyactive anode surfaces.
 2. The process of claim 1, wherein dissolvedalumina predominantly combines with oxidised fluorine ions to producealuminium fluoride and oxygen.
 3. The process of claim 2, whereindissolved alumina combines with monoatomic nascent fluorine formed byoxidation of fluorine ions to produce oxygen gas and partly dissociatedaluminium fluoride.
 4. The process of claim 1, wherein aluminiumoxyfluoride ions predominantly rather than oxygen ions are oxidised. 5.The process of claim 4, wherein aluminium oxyfluoride ions resultingfrom the combination of aluminium fluoride and alumina rather thanoxygen ions are oxidised on the electrochemically active surfaces intotransient aluminium oxyfluoride which decomposes into oxygen andaluminium fluoride.
 6. The process of claim 1, wherein the operatingtemperature of the electrolyte is below 900° C., preferably below 880°C., and even more preferably below 870° C.
 7. The process of claim 1,wherein the electrolyte contains cryolite and, in addition to cryolite,an excess of AlF₃ in an amount of at least 20 weight % of theelectrolyte, preferably between 25 and 35 weight % of the electrolyte.8. The process of claim 1, wherein the electrolyte further contains CaF₂and/or MgF₂.
 9. The process of claim 1, wherein said alloying metal ofthe nickel alloy is iron.
 10. The process of claim 1, wherein the outerpart of the anode comprises more than 75 weight % nickel, preferablybetween 85 and 95 weight % nickel.
 11. The process of claim 1, whereinthe outer part has an open porosity defining a high surface areaelectrochemically active surface, current being passed at a low currentdensity on the high surface area electrochemically active surface. 12.The process of claim 11, wherein part of said alloying metal of thenickel alloy dissolves into the electrolyte to form said open porosity.13. The process of claim 1, comprising circulating electrolytecontaining dissolved aluminium to constantly maintain dissolved aluminanear the electrochemically active anode surfaces.
 14. A cell for theelectrowinning of aluminium from alumina dissolved in a fluoride-basedmolten electrolyte operating at reduced temperature and utilisingmetal-based anodes comprising an alloy of nickel and an alloying metalhaving an outer part consisting predominantly of nickel which forms anelectrochemically active surface for the oxidation of ions, in which theelectrolyte contains AlF₃ in such a high concentration thatfluorine-containing ions predominantly rather than oxygen ions areoxidised on the electrochemically active surfaces, however, only oxygenis evolved, the evolved oxygen being derived from the dissolved aluminapresent near the electrochemically active anode surfaces.
 15. The cellof claim 14, wherein the temperature of the electrolyte is below 900°C., preferably below 880° C., even more preferably below 870° C.
 16. Thecell of claim 14, wherein the electrolyte contains cryolite and, inaddition to cryolite, an excess of AlF₃ in an amount of at least 20weight % of the electrolyte, preferably between 25 and 35 weight % ofthe electrolyte.
 17. The cell of claim 14, wherein the electrolytefurther contains CaF₂ and/or MgF₂.
 18. The cell of claim 14, whereinsaid alloying metal of the nickel alloy is iron.
 19. The cell of claim14, wherein the outer part of the anodes comprises more than 75 weight %nickel, preferably between 85 and 95 weight % nickel.
 20. The cell ofclaim 14, wherein the nickel alloy has a decreasing concentration ofsaid alloying metal towards the electrochemically active surface layer.21. The cell of claim 20, wherein the nickel alloy has a nickel metalrich outer part with a porosity defining a high surface areaelectrochemically active surface, said porosity containing cavitieswhich are partly or completely filled during use with fluorides of atleast one metal selected from nickel, said alloying metal and aluminium.22. The cell of claim 20, wherein the nickel metal rich outer partcomprises nickel metal and said alloying metal in a nickel/alloyingmetal atomic ratio of more than 3 where it reaches the electrochemicallyactive surface.
 23. The cell of claim 14, wherein the alloy of nickelwith said alloying metal has before use a nickel/alloying metal ratiobelow
 1. 24. The cell of claim 14, wherein the alloy of nickel with saidalloying metal has before use a nickel/alloying metal ratio of at least1, in particular from 1 to
 4. 25. The cell of claim 14, wherein thealloy of nickel with said alloying metal contains one or more additives,the alloy before use containing nickel with said alloying metal in atotal amount of at least 85 weight %, preferably at least 95 weight %,and the balance said additive(s).
 26. The cell of claim 24, wherein oneor more additives are selected from chromium, copper, cobalt, silicon,titanium, tantalum, tungsten, vanadium, yttrium, molybdenum, manganese,aluminium and niobium in a total amount of up to 10 weight % inparticular up to 5 weight %, of the alloy before use.
 27. The cell ofclaim 25, wherein one or more additives are catalytically active andselected from iridium, palladium, platinum, rhenium, rhodium, ruthenium,tin or zinc metals, Mischmetals and their oxides and metals of theLanthanide series and their oxides as well as mixtures and compoundsthereof in a total amount of up to 5 weight % of the alloy before use.28. The cell of claim 14, wherein before anodic polarisation the nickelalloy is covered with an integral oxide film obtainable by oxidising thealloy in an oxidising atmosphere.
 29. The cell of claim 14, wherein eachanode is a nickel iron alloy-based anode.